The invention relates to an SCR Top Connector Assembly for the articulating connection of a conduit, such as an offshore flowline or pipeline used in the petroleum industry, to a foundation subjected to differential motions of the conduit and structure. More particularly, this invention relates to a subsea pipeline with significant unsupported length, termed a xe2x80x9csteel catenary riserxe2x80x9d (SCR), which would utilize the SCR Top Connector Assembly to connect the SCR to a fixed or floating structure, including several parts that work together to provide fluid communication from the SCR to the platform piping system, to allow dynamic and relative motions of the SCR and the structure.
In the offshore oil and gas sector, recent developments in deeper water depths have demonstrated the need for improved solutions for the economic attachment of a flowline, or pipeline, to a structure, whether fixed or floating. Initial development utilized flexible pipe from the seabed to the floating platform; however, many operators have begun to favor the potential safety and savings offered by the use of steel catenary riser (SCR) configurations, wherein the pipeline is suspended for some distance off the seabed and connected to the structure, or floating platform.
The prior art assembly of SCR flexible joints greatly limit the approach corridors of flowlines due to the degree of dynamic angular movement that can be accommodated. Manufacturing and installation tolerances offshore in deep water leave little dynamic allowance in the prior art once the static offset of floating vessels are included. The angular limits of the prior art assembly pose difficulties for the design and construction engineers of offshore pipelines to ensure that the installation tolerances, fabrication tolerances, and operational conditions will not exceed the limits of the flexible joint and potentially lead to failure of the SCR. If the limits of the flexible joint are exceeded, all flexibility is lost and the SCR is exposed to very large bending moments resulting in dramatically and unpredictably shortened fatigue life, thereby leading to possible failure of the SCR below the flexible joint near the platform. Failure of an SCR in petroleum gas service or an SCR connector component without a viable safety shutdown valve would pose a very high risk of fire and loss of life as the gas in the pipeline (extending frequently 60 miles from the host platform) would be released at the base of the manned structure which contains sources of ignition. Although a gas leak may be more hazardous, failure of a SCR in petroleum liquid service would lead to a fairly large oil spill in open water since much of the oil in the pipeline would be siphoned out of the pipeline by the low pressure wake of the SCR falling to the seabed. Additionally, the oil would be expelled by expanding gases within the oil, as well as normal molecular diffusion.
It is an object of this invention to have features which greatly increase safety over present art by eliminating a risk of gas leakage at an offshore platform by allowing the use of normal and proven safety valves. This reduces the potential for fire, and eliminates potential oil leakage into seawater when used with oil lines. Current SCR flexible joints utilize elastomeric and metal laminations, which provide pressure containment. The same elastomeric materials serving as seals must absorb the full SCR vertical reactions while repeatedly being deflected under high vertical loads. The vertical loads can reach 100 tons due to the suspended riser weight, motions of the SCR subjected to continual environmental loading, and relative platform movements. Under cyclic loading, the elastomeric elements containing the fluids, under various conditions of temperature and pressure, may likely become a path for gas or oil leakage which would result in oil contamination of the surrounding seawater or leakage of gas at the base of the offshore platform. This would cause a gas plume and a risk of sinking the floating vessel or risk of fire to the structure overhead. If a semi-submersible were to sink at a corner, it would likely lose tendons and capsize. The industry considers systems with moving elastomeric parts to have maintenance or replacement requirements at some point in time and therefore leakage considerations are valid considerations with elastomers subjected to high cyclic compressive and shear loads acting as the sole safety barrier under pressure. Although platform piping valves can be closed on the platform side in the present art flexible joint, it is presently not considered possible to provide a safety block valve below the prior art flexible connector due to the high axial loads and high bending moments at the top of the SCR. These loads would be extremely taxing to the integrity of the valve and would not be a reliable safety feature.
A lower block valve, though not practical with present (prior art) equipment, would prevent elastomer leakage from causing the entire pipeline or SCR from back-flowing gas into the platform creating an uncontrollable hazard to life of platform personnel. In fact, the bending moments and related stresses are so high in the top section of the SCR, below the flexible connector, that specially fabricated tapered xe2x80x98stressxe2x80x99 joints are required to minimize the stress concentration factors in these installations. The stress intensification values are primarily due to the high bending moments resulting from the flexural high stiffness of the prior art SCR Top Connector to which the SCR is connected. The high bending moments are primarily a result of the high rotational stiffness of the laminated elastomeric elements being deflected laterally in shear while under high compression loads.
The replacement, or leakage failure correction, of the elastomers within the prior art assemblies is essentially impossible by the platform crew or by means that can be flown offshore to the site, or otherwise be effected in a short duration to reduce a prolonged platform hazard. It is a further object of this invention to eliminate elastomers from the multiple role duties of high load absorption, flexural cycling, and high-pressure containment of petroleum liquids and gases.
To correct fluid leakage of the prior art assemblies, it is necessary to remove the SCR and its associated pipeline from service by shutting in the platform and purging the section in order to provide a safe repair environment. It is also necessary to employ the use of costly offshore deepwater service equipment with sufficient lifting capacity to remove and re-attach a new assembly since the elements are not able to be replaced by the platform crew in a timely fashion in the prior art assemblies.
Prior art is described in part by the following patents:
U.S. Pat. No. 3,692,337 Flexible Coupling, Mischel; Howard T., San Diego, Calif. Sep. 19, 1972.
U.S. Pat. No. 3,952,526 Flexible Supportive Joint for Subsea Riser Flotation, Watkins; Bruce J., Rancho Palos Verdes, Calif. Apr. 27, 1976.
U.S. Pat. No. 5,791,695 Flexible Joint for Facilitating Bending of Tubular, Snider; David A., Hurst, Tex. Aug. 11, 1998.
U.S. Pat. No. 5,615,977 Flexible/Rigid Riser System, Moses; Charles J., Alvarado, Tex. Apr. 1, 1997.
U.S. Pat. No. 5,628,586 Elastomeric Riser Tensioner System, Arlt, III; Edward J., Arlington, Tex. May 13, 1997.
U.S. Pat. No. 4,105,266 Laminated Bearing with Plural Modulus Layer, Finney August 1978.
U.S. Pat. No. 4,759,662 TLP Marine Riser Tensioner, Peppel July 1988
It is a further object of the present invention to minimize the potential for fire risk and loss of life; oil spills, uncontrolled and extended leakage, high maintenance costs and pipeline/platform downtime duration.
It is another object of the present invention to provide a system with automatic shut-in safety block valve capability on each side of any non-metallic elements which may be subject to leakage.
It is a further object of the present invention to increase the allowable dynamic displacement angles to reduce the chance of bottoming out and causing premature SCR fatigue failure.
It is an additional object of the present invention to minimize the high stress levels which occur at the base of the prior art flex joint to eliminate the needs for specially fabricated tapered stress joints and provide extended fatigue service life of the SCR by reducing the top section fatigue moments.
It is yet another object of this invention to isolate the high loads due to the suspended risers from acting on flexible or elastomeric elements.
It is also an object of this invention to use load isolated swivels which convert pendular motions to rotary motion and allow system use when pressure and diameter restrictions prevent the safe use of flexible pipe.
It is also an object of the present invention to provide external means of dynamic high-frequency damping.
It is also an additional object of the present invention to allow essentially unlimited pipeline approach angle to the pre-installed attachment of steel catenary risers on platforms.
The present invention includes a mechanical joint assembly in which the load-absorbing base is composed of steel or alternate high strength components, providing a higher level of safety and spill prevention than is offered by prior art. The present invention further provides for the use of valving upstream and downstream of the only non-metallic maintenance item(s) without decreasing fatigue life. The present invention also reduces bending moments and resulting fatigue stresses at the SCR top by removing resistance to movement in all angular directions, with increased lateral angles to provide increased construction tolerances for the pipeline approach corridor. The present invention allows the use of increased angles of in-plane dynamic motion with elimination of large vertical SCR supported weight reactions from acting on elastomeric components.
The safety requirement, of preventing uncontrolled back feeding of the pipeline or SCR and thus averting fire and loss of life, is achieved by absorbing all reactions of the SCR prior to subjecting elastomers to pressure containment requirements. By isolating the high loads, downstream and upstream, remote or manual-operated, shut-off valves can be provided to fully isolate any leak within a short section without sacrifice of the system fatigue life. Preventing any block valve from absorbing the high SCR environmental reactions minimizes the chance of valve leakage or malfunction.
The long service outage and high maintenance costs are eliminated by providing the above noted block valves, limiting possible maintenance items to simple diver replacement components without the need for any offshore service vessels.
The present invention therefore separates the design requirements of high load control and that of allowable motion and flexure. The separation point may involve several means, which are described herein along with more specific details of the preferred embodiments.
In the preferred embodiment, increasing the allowance for the pipeline approach angles is accommodated in the lower base by providing dual orthogonal trunnions in addition to an axial swivel to compensate for installation rotational misalignment. A comparison can be made between the present invention and that of the prior art: when the normal construction tolerances are subtracted from the systems three to four times greater allowable dynamic angles are achieved by the proposed system than when compared to the prior art. This increase greatly reduces the chance of exceeding the flexure rates associates with high stress levels when pressure limits are exceeded.
Doubling the load carrying capacity is achieved in the present invention by providing a robust design with heavy cross sections and gradual section transitions in the base to minimize stress levels and, stress intensification factors, thereby providing improved fatigue resistance and load handling.
Elimination of specially prepared tapered stress joints is achieved by the omission of elements which resist the angular motion of the SCR by high-bending moments, as is in the case of the prior art assembly, and the substitution of either low friction, higher-paired pivot systems, bearings, bushings, low-friction coatings, or ultra smooth surfaces. Systems above the base are provided to absorb the angular rotation without tensile loading and with low rotational stiffness.
The Top Connector Assembly may include an optional damping system to curtail high frequency motion. The damping system is subjected to the high SCR reactions and may be maintained, or replaced, without shut-in of the system by a diver and small platform-mounted equipment. Because the fluid medium for damping may be seawater, or other benign fluids, failure of the system in any way does not constitute an emergency. The maintenance could include only the installation of a new damper.
The benefits afforded by the objects of this invention, as defined, clearly address safety concerns of the prior art devices and increase the operational limits and reliability by allowing greater SCR motions with less constraint while eliminating present concerns of installation tolerances. These features in turn allow for measurably larger SCR storm-induced dynamic movement angles with safety.
The assembly of the preferred embodiment of the present invention is composed of several principal parts which make up the load absorbing base and the flexible assembly:
(A) A pressure carrying body which is attached to the uppermost portion of the SCR riser. This component incorporates a pair of male trunnions, which are structurally connected to the pressure containment body and located at opposite sides of the pressure-carrying body, which is an extension of the SCR pipe. The trunnions include features of higher paired rolling motion that provide essentially frictionless motion. Alternate arrangements when friction is less significant due to light SCR reactions include bearings, or lubricated bushings for the trunnions. Rotary motion of the pressure containment body is provided by interior coatings or other means to prevent locked-in torsional stresses during installation;
(B) A trunnion adapter, which consists of a female trunnion on the interior surface, which mates with the pressure containment body and additionally contains a second xe2x80x9couterxe2x80x9d male trunnion pair, as described above, located on the outer surface and on an orthogonal plane (a perpendicular plane) to the interior trunnions;
(C) A foundation receptacle that is attached to a foundation of the structure and accepts the outer surface trunnion pair of the second part. The trunnions act in the principle of a universal joint and allows movement in any angle;
(D) Rounded-knife edge pivot, bearings, or bushings may be utilized to provide for rotation of the trunnions to reduce the friction to the degree required for the application;
(E) An optional damper, which can be utilized to absorb high frequency motions and prevent resonance of an undesirable mode due to dynamic excitation of the environment may be included in the assembly as a system approach solution;
(F) A series of swivels converting pendular to rotational motion, flexible pipe, or other means of the prior art, is incorporated into the assembly above the load absorbing base to provide flexure. The load requirements for the swivels and/or flexible pipe are significantly lower because the external loads have been absorbed by the trunnion assembly and associated components;
(G) The swivel or flexible pipe assembly may be preferentially protected at each end with automatic or manual valving at the upstream and downstream ends for automatic and manual shut-in safety without compromising the fatigue life of the system.